In three dimensional vision (3D) systems, a primary task is to measure the x, y, and z components of various key points on a sample under inspection. If the location of the various key points falls within prescribed limits, the sample passes the inspection.
One means of doing 3D vision involves the use of active optical triangulation which works as follows: a light source, such as a laser, produces a thin beam which is focused onto a sample under inspection. The resultant illuminated spot on the sample is imaged onto an optical photo-sensor, such as a charge coupled device (CCD) or position sensitive device (PSD). The height of the sample may be determined from a knowledge of the spot location on the image plane of the sensor. By moving the apparatus, consisting of the light source and sensor, relative to the sample, the height of various points on the sample may be measured.
One application of 3D vision is the measurement of location and height of leads on electronic integrated circuit chips (ICs). In a typical machine, the 3D sensor, consisting of the light source and photo-sensor is mounted to an X-Y motion stage which moves the sensor over a rectangular region. ICs are moved into this region and inspected by the sensor which is moved over the ICs by the motion stage. The inspection consists of sampling the location of key features of the IC such as the location of each of the lead tips, the lead width, the lead height, the distance between leads, and other features.
One of the issues associated with the 3D inspection process is the minimization of the inspection time and the amount of data needed to properly characterize the IC. Restricting the discussion to the case of inspecting only the leads of the IC, the sensor would first be moved to the region of the inspection zone bounding the extremities of the IC. Inspection would then consist of sampling various points in this region to determine the presence of the IC leads and their geometrical characteristics.
This invention is concerned with a method of restricting the search region for the multiplicity of IC leads and the amount of data required to determine the lead characteristics.
The search is typically carried out by a combination of two methods. In the first method (mechanical scanning), the sensor is positioned at an x,y location (the x and y coordinates are associated with the planar, rectangular search region), and the sensor determines the z-height associated with this location. The sensor is then moved to a new position and the z-height measured. A lead is determined to be present at a given location if the value of z falls within a certain range. The edge of a lead is determined from a transition in z values of neighboring points in the search region.
In the second method (electronic scanning), the sensor is held fixed and the light beam is moved using a typical light deflection means. The z-height is determined at each of the several x,y deflection locations of the illuminated spot in the search zone. Typical light deflection means include moving mirrors and acoustical-optical deflectors.
In many applications such as vision-based inspection and metrology, the uncertainty in the position of the feature of interest necessitates either a relatively slow search by mechanical scanning intertwined by a fast search in laser beam deflection, or only a fast search confined to laser beam deflection with the sensor traversing along an assumed nominal path.
Both of these searches, particularly search by mechanical scanning is detrimental to the throughput in many applications.
The object of the present invention is to remove the need for the aforementioned searches.
The present invention is discussed, particularly in the context of data acquisition for quad flat pack devices, a currently popular IC package containing up to several hundred leads.
The leads on quad flat packs are located on all four sides of the body. Each side has a multiplicity of leads extending out from the body parallel to each other and approximately the same length. The ends of the leads (furthest from the chip body) are referred to herein as the lead toes.
The object of the present invention is to most economically find the location of the nominally straight line joining the lead toes for each of the four chip sides. This information is used to optimally control the x, y motion and the light deflection mechanism.